Device for adjusting an operating point of a magnetic field sensor

ABSTRACT

The invention relates to a device for setting an operating point of a magnetic field sensor having a periodic characteristic, in particular for a device for detecting a magnetic field and/or flux, having a SQUID as magnetic field sensor and a control unit which is connected downstream of the SQUID, has a control time constant (t) and has a feedback loop which acts on the SQUID and is designed such that it is active about a number of operating points of the SQUID, where flux quantum pump means are provided which are assigned to the SQUID, have a signal generation unit for generating a control and/or regulation signal for the SQUID and are designed such that, in order to pump at least one flux quantum into and out of the SQUID, a signal form of the control and/or regulation signal, generated by the signal generation unit, is different and, referred to a rising and a falling edge of a signal form, is unsymmetrical, where in each case only one of the edges of a signal form is short referred to the control time constant.

BACKGROUND OF THE INVENTION

[0001] (1) Field of the Invention

[0002] The present invention relates to a device for setting anoperating point of a magnetic field sensor. Such a device is known forexample from the German Laid-Open Specification 196 06 655 A1and has, asessential magnetic measurement element, a SQUID which is held at a fixedoperating point by a control loop (FLL=Flux Locked Loop).

[0003] (2) Description of the Related Art

[0004] The problem which underlies the cited generic prior art inmagnetic flux measurement by means of SQUID can be illustrated withreference to the signal diagram shown schematically in FIG. 4 (flux θ asinput parameter, output signal U of the SQUID electronics): The outputsignal is periodic with spacings of a flux quantum θ₀, so that themeasurement and compensation in particular of highly fluctuatingmeasured signals is problematic; as can be clearly seen from FIG. 4, theanalog flux control illustrated by the control loop in particularincluding in FIG. 1 of DE 196 06 655 operates linearly only within aninput signal range of half a flux quantum.

[0005] This problem is solved by the cited specification from the priorart in that the analog control loop is additionally supplemented by adigital control component, so that the counting of the flux quanta ofthe overall arrangement is unambiguous and no flux quanta are lost. Aresetting of the flux quanta is effected, according to this prior art,by opening the analog control loop or by means of a so-called clampingdevice.

[0006] Nevertheless, in practical operation, in particular during use inunshielded environments and in the case of rapidly changing measuredsignals, the operation of a generic, hybrid technology (that is to sayone which has analog and digital control elements) is not withoutproblems: the low slew rates provided on account of the system areexceeded even in the case of highly magnetic interfering influences, andthe slow resetting of the generic technology prevents it from beingpossible to follow rapidly changing measured signals.

[0007] In addition, the rapid signal changes bring about controldeviations in the control unit used, and these control deviations causemeasurement inaccuracies.

SUMMARY OF THE INVENTION

[0008] It is therefore an object of the present invention to develop ageneric device for setting an operating point with respect to its usablebandwidth and/or the usable control range to the extent that it ispossible to compensate, reliably and without delay, in particular evenover a measurement range comprising a number of flux quanta.Accordingly, a device is to be provided which can be reliably operatedeven in environments having high magnetic control influences and/orinsufficient shielding conditions.

[0009] In an advantageous manner according to the invention, the fluxquantum pump means ensure that, without there being any need to fearcontrol influences of the control unit, it is possible for flux quantato be pumped into and out of the SQUID by the signal generation unitusing characteristic signal forms, as a result of which the controlrange accordingly expands by this possible number of flux quanta.

[0010] In particular, the short edge according to the invention (rise inthe case of pumping in, fall in the case of pumping out) means that, bycomparing short edge durations with the control time constant of thecontrol unit, the control loop cannot follow this signal change andhence does not compensate the quantum flux signal; this then leads (cf.FIG. 4), by virtue of the pumping, to a period-based or fluxquantum-based movement over the periods and hence over the θ₀/2 controlrange of the traditional control loop being possible.

[0011] As a result, the analog flux control is possible in anuninterrupted manner, that is to say the dead times or delay times forthe digital setting and resetting means, which are known from the priorart and needed therein, do not occur, so that in an advantageous manneraccording to the invention the device according to the present inventionis also able without any problems to follow rapid signal changes of aninput signal that is to be measured and moreover also allowsconsiderably higher slew rates.

[0012] Particularly in the case of pumping out, the resetting known fromDE 196 06 655, which is effected by opening the control loop, is thusunnecessary; rather, a flux quantum is accordingly pumped out (or pumpedin) in particular when the amplitude of the flux pulse generated by theflux quantum pump means or the associated signal generation unit isgreater than a flux quantum. Dead times are avoided.

[0013] It has proven to be particularly preferred to configure theunsymmetrical signal forms according to the invention, for bringingabout the pumping in or out, in a triangular manner, where either (ineach case compared to the control time constant) a short rising edgelies opposite a long falling edge or, in the case of pumping out, ashort falling edge follows a slow rising edge.

[0014] In the preferred design embodiment, the signal generation unitaccording to the invention, which otherwise in a known manner generatesthe signal or pulse forms according to the invention, is coupled intothe flux control loop, and in particular the control or regulationsignal generated according to the invention acts on the voltage fluxconverter means assigned to the flux control loop, which voltage fluxconverter means are typically embodied as a coil and generate thedesired flux signal for SQUID from a current or voltage signal. As aresult, there is thus a closed control loop which overcomes inparticular the disadvantages of the hybrid control loop described abovefrom the generic prior art, said prior art control loop not beingcompletely closed, and in particular, for the reasons described above,does not require any opening or suchlike measures of a control loop forresetting purposes, as is required in the prior art.

[0015] In the practical embodiment, it has additionally been proven tobe preferred for the PID controller provided according to onedevelopment to be configured with a typical difference or servoamplifier, which is used in a known manner to generate the measured oruseful signal; in an advantageous manner according to one development,an integrator formed in this way is also designed for higher than thefirst order. This then also leads to it being possible to avoid theinaccuracies in the measurement response—brought about by rapid signalchanges—and additionally to it being possible to detect greater fluxchange rates.

[0016] A number of current SQUID devices are suitable for use in thecontext of the present invention, for instance the rf-SQUID or dc-SQUIDwhich are otherwise known, having two or more Josephson contacts.

[0017] In particular, it is also within the scope of the presentinvention to configure the signal form according to the invention forpumping in or pumping out such that more than one flux quantum is pumpedwith one signal pulse; this preferably takes place by correspondinglyconfiguring the amplitude of the signal.

[0018] It is also within the scope of the present invention to assign tothe control unit according to the invention, besides analog controlmeans, also digital control means, so that a hybrid flux control loopcan also be formed within the context of the invention. By contrast withthe prior art, however, the flux quantum pump means provided accordingto the invention are a direct part of the hybrid flux control loop andthus permit independent and in particular also delay-free operation ofthe analog and digital control means.

[0019] As a result, compared with the prior art, there is thus aconsiderably improved arrangement having an actuator, which still allowsreliable and interference-free measurement operation particularly inmeasurement environments with unfavorable shielding or in the case ofvery rapid and pronounced signal changes of the measured signal. Thepresent invention can surprisingly be implemented in a simple mannerwith a low outlay on design.

[0020] Further advantages, features and details of the invention emergefrom the following description of preferred examples of embodiments andwith reference to the drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 shows a schematic block diagram of the present inventionaccording to a first preferred embodiment;

[0022]FIG. 2 shows a signal diagram of an unsymmetrical signal formgenerated by the signal generation unit in FIG. 1, for pumping a fluxquantum into the device;

[0023]FIG. 3 shows a signal diagram similar to FIG. 2, for pumping aflux quantum out of the SQUID and

[0024]FIG. 4 shows a general signal diagram which illustrates theconnection between magnetic flux as input signal or measured signal of aSQUID and the output voltage signal U which is periodic with a fluxquantum θ₀.

DETAILED DESCRIPTION

[0025]FIG. 1 schematically shows the design and functioning of thepresent invention. A control unit delimited schematically by theboundary line 10 has, in a known manner, a servo or difference amplifier12 which is assigned to a SQUID 18, generates a feedback signal asreaction to a signal difference on the SQUID and leads a feedback coil16 over a coupling resistance, which feedback coil then, with a controltime constant t inherent in the control unit, allows compensation of thedetected signal change by generating a corresponding compensation flux.To this extent, the arrangement shown corresponds to the control looparrangement shown in FIG. 1 of DE 196 06 655 A1, to which reference ishereby made for further details of the practical embodiment and whichallows the person skilled in the art to readily implement it inpractical terms.

[0026] In an inventive manner, the control loop now additionally hasassigned to it a signal generation unit 20 as flux quantum pump which,again over a schematically illustrated coupling resistance, provides thefeedback coil 16 with a flux quantum pump signal or a pump pulse whichaccording to the invention is formed to pump a flux quantum into or outof the SQUID 18 and by virtue of its characteristic signal form, as willbe described below, remains uninfluenced through the control loop.

[0027] In more precise terms, the signal form generated by the signalgeneration unit 20, for pumping in one (or more) flux quantum(quanta),has an unsymmetrical contour in the flux/time diagram of FIG. 2 and isconfigured such that the rise time t₁, of the rising edge in the pumpsignal is short referred to the control time constant of the controlloop 10; by contrast, the duration of the falling edge t₂ of the pumpsignal shown in FIG. 2 is long compared to the control time constant t.This means that, by applying to the feedback coil 16 a signal having thepulse form shown in FIG. 2, the control loop 10 cannot follow the steepedge rise t₁, of the rising edge, and hence the control loop thereforecannot compensate the additional flux rise brought about by the controlsignal; this then leads to the signal jumping to the right by one fluxquantum in the signal diagram in FIG. 4, and to this extent the controlunit which additionally has the flux quantum pump can follow the signalalso over the signal width up to now of only half a flux quantum. Thefalling edge shown in FIG. 2 is by contrast in turn compensated by thecontrol loop within the control time constant, so that the SQUID remainsat the increased quantum flux level.

[0028] In practice, rapid rise times in the region of about 10 ns canthus be implemented, with the fall times typically being greater by afactor of 2. In order to obtain and transmit neat signal forms also inthese ranges, lines adapted for high frequency are furthermorepreferably provided according to the invention.

[0029]FIG. 3 describes the case that is similar to but the reverse ofFIG. 1 and thus a pumping of a flux quantum out of the SQUID. The risingedge, which is controlled with a slow rise time t₃, is compensated bythe control loop, but the falling edge which is short at t₄ is too shortfor a reaction of the control loop having a control loop constant t, sothat a flux quantum is pumped out of the SQUID.

[0030] By virtue of these two modes, the possibility is in particularprovided of achieving a predetermined setting of the SQUID to anoperating point of the periodic characteristic in a simple and above allinterruption-free manner, something which traditionally could only takeplace by opening the analog control loop with the time drawbacks broughtabout thereby (cf. the generic prior art). In an otherwise known manner,an analog control unit provides the information required for thesetting.

[0031] While the example of embodiment described firstly describes themovement by in each case individual flux quantum steps by the fluxquantum pump according to the invention, the present invention is notrestricted to the pumping in and out of individual flux quanta butrather the simultaneous pumping of a number of flux quanta in bothdirections is possible in particular by virtue of a suitable (amplitude)configuration of the control signals in a manner analogous to FIG. 2 andFIG. 3.

1. A device for setting an operating point of a magnetic field sensorhaving a periodic characteristic, in particular for a device fordetecting a magnetic field and/or flux, having a SQUID as magnetic fieldsensor and a control unit which is connected downstream of the SQUID,has a control time constant (t) and has a feedback loop which acts onthe SQUID and is designed such that it is active about a number ofoperating points of the SQUID, characterized by flux quantum pump meanswhich are assigned to the SQUID, have a signal generation unit forgenerating a control and/or regulation signal for the SQUID and aredesigned such that, in order to pump at least one flux quantum into andout of the SQUID, a signal form of the control and/or regulation signal,generated by the signal generation unit, is different and, referred to arising and a falling edge of a signal form, is unsymmetrical, where ineach case only one of the edges of a signal form is short referred tothe control time constant.
 2. The device as claimed in claim 1,characterized in that the signal generation unit is designed such that,in order to pump the at least one flux quantum in, the signal form has arising edge with a rise time (t1) that is short with respect to thecontrol time constant (t) and a falling edge with a fall time (t2) thatis long with respect to the control time constant.
 3. The device asclaimed in claim 1 characterized in that the signal generation unit isdesigned such that, in order to pump the at least one flux quantum out,the signal form has a rising edge with a rise time (t3) that is longwith respect to the control time constant and a falling edge with a falltime (t4) that is short with respect to the control time constant. 4.The device as claimed in claim 1, characterized in that voltage fluxconverter means, in particular coil means, for the SQUID are connecteddownstream of the signal generation unit, where the voltage fluxconverter means are preferably also connected downstream of the controlunit and act as part of the feedback loop.
 5. The device as claimed inclaim 1, characterized in that the flux quantum pump means form a closedcontrol loop with the signal generation unit and the control unit. 6.The device as claimed in claim 1, characterized in that the control unithas a servo amplifier which acts in particular as a PI^(N) or PI^(N) Dcontroller, where N≧1, which servo amplifier is preferably designed toprocess also integrators of higher than the first order.
 7. The deviceas claimed in claim 1, characterized in that the SQUID has an rf-SQUIDand/or a dc-SQUID, which has more than two Josephson contacts.
 8. Thedevice as claimed in claim 1, characterized in that the signalgeneration unit is designed such that the signal form can be generatedand configured such that more than one flux quantum is pumped into orout of the SQUID.
 9. The device as claimed in claim 1, characterized inthat the device has analog and digital control means for configuring ahybrid flux control loop over the entire range of the characteristic ofthe SQUID, where the flux quantum pump means are set up as part of thehybrid flux control loop such that they permit independent operation ofthe analog and digital control means.
 10. A method of pumping at leastone flux quantum into and out of a SQUID wired to a control loop, wherethe control loop has a control time constant (t) and acts on the SQUIDand is designed such that it is active about an operating point of theSQUID, characterized by the steps: generation of a control and/orregulation signal for the pumping in, having a first signal form whichhas a rising edge that is short with respect to the control timeconstant and generation of a control and/or regulation signal for thepumping out, having a second signal form which has a falling edge thatis short with respect to the control time constant.
 11. The method asclaimed in claim 10, characterized in that the generation of the controland/or regulation signal comprises the introduction of a current signalinto a feedback coil which is preferably part of the control loop. 12.The method as claimed in claim 10 characterized in that the controlrange of the control loop is set by the control and/or regulation signalsuch that an analog control range is made possible over a number of fluxquanta.